especially when they're not expected in the normal course of business.

I get them from people I interview for positions at my organization, and that's nice. But because that seems to be pretty standard operating procedure, it doesn't mean much. But when I get them from a client or someone I've interacted with outside of my usual course of business, it has an outsize impact on me. I remember them, and want to help them, or will be willing to go to bat for them in the future. Part of it is that I remember the note the wrote, but its even more fundamental than that, like I now subconsciously associate them with being a good human being.

Over the last few years I've tried to do that a little more, writing notes to select people a few times a year. I don't have any evidence to back it up, but I think it helps how they view me as well. If I had to distill it down, it would be something pity like "Most people want to help you, you just have to give them a reason to remember you when the time comes for them to give you a hand".